Study: Cellulosic Ethanol Production Might Harm Climate

Biofuels made in the Midwest from corn stover -- the leftovers of harvested corn plants -- might be worse for global warming than gasoline in the short term.

The findings are in a recent study that's casting doubt on the greenhouse benefits of cellulosic ethanol.

Bales of corn stover, which can be used to create cellulosic ethanol, sit in a field.

Credit eXtension Farm Energy/Flickr

The peer-reviewed study was conducted by researchers at the University of Nebraska Lincoln and appears in the journal Nature Climate Change. It modeled what the impact on carbon emissions would be if you took corn stover from fields across the Midwest to make ethanol. Usually, corn stover is left on harvested fields to improve soil health, but removing it would create more carbon dioxide emissions.

Corn ethanol is big business in the Midwest, but almost all the fuel is made from the grain, not the leftover stalks. Cellulosic ethanol is only produced on a small scale today, but two large cellulosic ethanol plants are being built in Iowa that would use corn stalks and other residue as their main fuel source.

The researchers from UNL found that fuel made from corn stover ultimately accounts for more carbon dioxide emissions across its lifecycle than gasoline in the first five years of production. They predict emissions will drop over time, but not enough to meet federal goals.

Cellulosic fuels are required to cut carbon emissions by at least 60 percent compared to gasoline in order to meet federal standards. The study suggests cellulosic fuels from corn stover, the kind that would come from those brand new plants in Iowa, may fail to meet that standard.

The Renewable Fuels Association called the study flawed, in part because it supposes farmers remove all of the corn stover from a field, when they are more likely to remove only about 50 percent.

Adam Liska, the lead author of the study and a professor at UNL, said by e-mail that models were run at three levels of residue removal, but it didn’t change the outcome because the study considers the carbon impact per gallon of ethanol, not per acre of cornfield. If you only remove 50 percent of crop residue for ethanol, you end up gathering it from a larger area because it takes a certain amount of material to make a gallon of cellulosic fuel. Gallon per gallon, Liska says, the impact on emissions is the same.

“Each of these removal levels produced the same CO2-intensity per unit of biofuel; this is a central finding of our research,” Liska wrote. “Which means it is likely that no matter what level of residue is removed, the carbon intensity stays the same.”

The Union of Concerned Scientists (UCS), is critical of using corn grain for ethanol but supports making cellulosic fuel from corn stover, along with a mix of other sources like switch grass. Jeremy Martin, a biofuels analyst with UCS, says the UNL study has not changed the organization’s position but it illustrates a central problem with taking stover for ethanol: the loss of carbon in the soil.

Crop residue helps rebuild soil carbon, which is an important part of soil quality. That doesn’t happen if it is taken away, and the soil can eventually suffer as a result. Previous studies, Martin says, have looked at how much carbon loss is acceptable

“But what (this study is) saying is it may be acceptable,” Martin said. “But what is the impact on climate? What is the impact on total CO2 emissions?”

There are ways to manage both carbon loss in the soil and the overall footprint of cellulosic ethanol. Martin says, and the study points out, farmers can make up for some of the lost carbon by planting cover crops or spreading manure or compost. Biorefineries can limit the greenhouse impact of corn stover ethanol by burning a byproduct from the production process to replace coal or natural gas – a point critics say the UNL study should have accounted for.

The study definitely gives a different perspective on cellulosic ethanol, but it may not do anything to change the trajectory of advanced biofuels. The EPA, which administers national ethanol policy, says the study is not relevant to its analysis of carbon emissions from biofuels.